The present invention relates generally to molds for curing pneumatic tires and more particularly to a method of making a segmented mold for curing tires.
It is well known to cure a tire in a mold having a plurality of matrix segments arranged within a mold shell. The matrices are supported on guide pins carried by the upper and lower mold shell halves, with the guide pins being disposed at an angle to the general axis of the matrix assembly. The matrix segments are spring loaded to move axially relative to the mold shell halves whereby when the matrix assembly is opened, the matrix segments are automatically moved axially and circumferentially to open positions for the reception or release of a tire. Such a construction is disclosed in several U.S. patents including MacMillan U.S Pat. No. 3,778,203 and Gulka U.S. Pat. No. 5,676,980. In such segmented molds, the upper and lower matrix segment assemblies define the configuration of the tire. The configuration of the tire being cured includes an outer peripheral portion wherein there is defined a tread pattern. Generally, in the case of known segmented molds a plurality of matrix segments are arranged circumferentially to define the configuration of the tire to be cured with the grooves in tire tread pattern being generated by ribbing formed in the matrix segments as such segments are cast. The tread design is generally defined by a number of pitches which can be equally divided into the number of matrix segments. If, however, a tire tread design calls for a number of pitches which cannot be divided equally into the number of matrix segments, it is necessary to provide complex casting patterns for the ribbing to accommodate the bridging of pitches at the juncture of adjoining matrix segments. The cost of casting such ribbing is much higher than the cost of casting conventional matrix segments utilizing a number of pitches which can be equally divided into the number of matrix segments.
MacMillan U.S. Pat. No. 5,120,209 discloses a tire curing mold employing a plurality of inserts equal in length to an individual pitch which are secured to a plurality of matrix segments. Each insert is formed with ribbing defining an increment of the complete tire tread pattern and the adjoining inserts match one another in defining such complete tire tread pattern. The individual inserts of MacMillan U.S. Pat. No. 5,120,209 are divided equally into the number of matrix segments. For example, with twelve matrix segments, six individual inserts are utilized with each matrix segment. A problem arises, however, where an odd number of inserts are necessitated by the configuration of a particular tread design, for example, where twenty-nine individual inserts are carried by eight matrix segments to define the tire tread pattern. Clearly, twenty-nine can not be equally divided by eight.
The present invention is directed to a method of making a segmented mold for curing tires utilizing a plurality of pitch-defining inserts that collectively generate a tread pattern wherein the number of individual inserts cannot be evenly divided by the number of matrix segments, e.g. where twenty-nine individual inserts are utilized with eight matrix segments.
In carrying out the method of the present invention, a first number of matrix sections are employed with respect to a second number of pitch defining inserts which cannot be equally divided by the number of matrix sections to collectively form a desired tire tread pattern. By way of example, twenty-nine inserts are supported by eight matrix segments. With this example, certain of the inserts will necessarily be disposed at the junctures of the matrix segments. The method of the present invention includes the steps of securing inserts to the entire periphery of joined-together matrix sections, except where an insert is to be positioned at the juncture of two adjoining matrix segments, to thereby form an empty space that extends over the side edges of each such juncture. The matrix sections are then separated along such juncture. A complete insert is secured to adjoining matrix sections within the empty spaces. Next, the portions of the inserts that overhang the side edges of the matrix sections are trimmed to conform to the surfaces of such side edges. The incremental portions of the inserts collectively generate the complete tire tread pattern, since the grooving formed by the ribbing of each insert matches the grooving of the adjoining insert. The matrix segments are then arranged in the mold shell with the adjoining inserts collectively defining the complete tread pattern of a tire to be cured. The upper and lower mold shell halves are closed with uncured rubber disposed therebetween. The rubber is cured so as to cure and define the tire. Finally, the mold shell is opened and the cured tire removed from the matrix assembly.
The method of the present invention provides a segmented mold having a number of pitch-defining inserts which cannot be equally divided by a number of matrix segments to be constructed in an efficient, cost-saving manner. Such mold also produces a clearly defined tire tread pattern that can be replicated over a long service life of the mold.
Further advantages afforded by the method of the present invention will become apparent from the following detailed description, when taken in conjunction with the accompanying drawings.